The rigorous thermodynamic theory of the pressure coefficient of interfacial tension, for systems containing a surfactant film at a liquid—vapor interface or a liquid—liquid interface, is developed. It is shown that a thickness parameter, λ 21, can be obtained from this measurement. λ 21 is the thickness of the region from which the major components of the respective bulk phases are excluded. In Gibbsian terms, it is the distance between the Γ 1 = 0 and Γ 2 = 0 dividing surfaces, when substance 3 is surface-active. Experiments in which the theory may be applied are discussed, particularly the measurement of liquid—vapor interfacial tension in the presence of a lipid film at its equilibrium spreading pressure. The physical interpretation of Γ 21, on a microscopic scale, is discussed.